Measuring sensors for the transcutaneous monitoring of the blood gases O.sub.2 and/or CO.sub.2 have been disclosed. It has also been disclosed, in DE-C3-21 45 400, for such measuring sensors, which are also called measuring probes or, for brevity, sensors, to heat electrically a metal body which acts as a reference or counter electrode, and to maintain at a constant temperature via a control device. The electrical heating has two functions in this. On the one hand, the warming generates hyperaemia of the skin and tissue underneath the measuring probe and, on the other hand, maintenance of a constant temperature of the measuring probe stabilizes the properties of the physical or electrochemical measuring elements contained therein. In addition, under certain conditions, the heat input required to maintain the temperature of the measuring probe can be used to monitor the relative local blood flow; compare the book by Huch, Huch and Lubbers: Transcutaneous pO.sub.2 ; Thieme Verlag Stuttgart-N.Y. 1981. In the same book, on pages 101 to 107, sensors and measuring devices for the transcutaneous monitoring of the partial pressure of oxygens, pO.sub.2, are described, in which the oxygen is measured by polarography, and on pages 78 to 80 it is pointed out that an adequate sensor temperature is necessary for validity of the transcutaneous pO.sub.2 monitoring. The minimum temperature for this purpose is regarded as being 43.degree. C.; temperatures up to 45.degree. C. are normally used.
In order to avoid skin damage at these temperatures, it is necessary from time to time to place the measuring sensor elsewhere on the surface of the skin; about every 4 hours at an operating temperature of 43.degree. C. This circumstance demands attention and supervision by reliable personnel, which, experience has shown, cannot be assumed in the domestic milieu. A procedure which is practised in order to be able, nevertheless, to carry out such monitoring entails use of two sensors which are alternately heated for a certain time and used for pO.sub.2 measurement; compare the article by PETER, H. J.: Holter Monitoring Technique in a Comprehensive Approach: Ambulatory Monitoring of Sleep Apnea; in Holter Monitoring Technique, Hergb.: HOMBACH/HILGER; VERLAG SCHATTAUER Stuttgart-N.Y. 1985, page 134/135. However, the disadvantages of this solution are that two sensors are required, which leads to difficulties, especially with small children, and that there is a need for complicated electronic means to switch over between the measuring probes at the preset time intervals. In addition, problems derive from the identical calibration of the two sensors, which is necessary.
Measuring problems for the transcutaneous monitoring of carbon dioxide (CO.sub.2) are described in, for example, DE-A No. 1-29 11 343 and DE-A No. 1-32 32 515. These measuring sensors contain pH measuring electrodes which measure the pH in a thin layer of an electrolyte solution which is enveloped by a CO.sub.2 -permeable membrane and is subject to gas exchange, via diffusion of CO.sub.2 gas, with the material which is to be measured. In this, the pH is measured either with a known glass electrode, as has been known for a long time in blood gas analysers, for example in the embodiment in DE-A No. 1 29 11 343, or more recently, with a special iridium/iridium oxide electrode, as described in DE-A No. 1-32 32 515.
It is desirable, and in critical cases necessary, to monitor both the oxygen and the carbon dioxide. Accordingly, DE-A No. 1-23 05 049 discloses the proposal, which is obvious per se, to combine the sensors for pO.sub.2 and pCO.sub.2 spatially in one sensor housing. This proposal has now led to combined pO.sub.2 /pCO.sub.2 sensors which must likewise be heated to at least 43.degree. C. to ensure satisfactory pO.sub.2 measurement. In addition recalibration at defined time intervals is necessary for the part measuring pCO.sub.2, which is effected, for example as specified in DE-A No. 1-23 05 049, by a glass electrode.
The invention is based on the problem of providing a combined sensor of combined measuring probe for the simultaneous and continuous transcutaneous measurement of carbon dioxide and oxygen in the blood for a prolonged period, for example throughout a day or a night, using a single sensor for monitoring, without it being necessary to interrupt this monitoring to resite the sensor, to switch over to another sensor, or for calibration procedures.
There is a need for a solution to this problem in, for example, the monitoring of patients with nocturnal breathing disorders or of children threatened by the socalled sudden infant death syndrome (=SIDS). This is because it has emerged that, where possible, such patients should also be monitored in their natural surroundings in the domestic milieu, whether for more accurate diagnosis of their disorder or in a period of transition from clinical supervision to the domestic milieu in which there appears to be an indication, for safety reasons, for the monitoring which was carried out in the hospital to be continued at home to a certain minimal extent. However, it must be possible even for non-experts to operate a measuring device for monitoring of this type in the domestic milieu without fear of unreliable measurement or risk to the patient.